Systems employing data storage elements include, for example, video cameras and image displays. Such systems employ an addressing structure that provides data to or retrieves data from the storage elements. One system of this type to which one embodiment of the present invention is particularly directed is a general purpose flat panel liquid crystal display whose storage or display elements store light pattern data. Flat panel-based display systems present a desirable alternative to the comparatively heavy, bulky, high-voltage cathode-ray tube-based systems.
A typical flat panel liquid crystal display comprises liquid material contained between two glass panels that are separated by a few microns and sealed together at their edges. The viewing area defined by this structure is divided into a multiplicity of display elements or "pixels." The optical characteristic of each pixel are determined by the electrical potential placed across the thickness of the liquid crystal material at the pixel location ("pixel gradient"). A pair of transparent electrodes, deposited onto the interior surface of each glass panel at each pixel, face each other across this thickness and form a capacitor. These terminals allow each pixel gradient and the resultant pixel optical characteristics to be independently manipulated. The pixels are spaced far enough apart from one another so that a first pixel gradient will not significantly affect the pixel gradients of the neighboring pixels.
Various schemes have been devised for allowing each pixel gradient to be set independently. In currently available active matrix liquid crystal arrays there is, generally, a thin film transistor ("TFT") connected to the pixel capacitor at every pixel. This TFT is typically strobed "on" by a row driver line at which point it will receive a value from a column driver line. This value is stored on the pixel capacitor until the next row driver line strobe. This method requires over 250,000 TFTs for a monochrome 512.times.512 pixel display and more than 750,000 TFTs for a color display. Because the failure of a single TFT is noticeable to a viewer, the requirement of having a TFT for each pixel drives up the cost of the display.
U.S. Pat. No. 4,896,149 describes the construction and operation of an alternative type of active matrix liquid crystal array, named a "plasma addressable liquid crystal" or "PALC" display. This technology avoids the cumbersome and restrictive use of a thin film transistor for every pixel. As in other flat panel displays, liquid crystal material is contained between two closely spaced glass panels. Each pixel is formed by a small area (typically on the order of a 0.25 square millimeter) of the liquid crystal material positioned between a first surface of a thin, impermeable dielectric barrier ("the barrier"), which comprises one of the two closely spaced glass panels and a photo-patterned conductive surface formed onto the other glass panel. Contacting a second surface of the barrier, which is opposed to the first surface, is a gaseous medium comprising a gas stored in a channel. The gaseous medium may be selectively switched from a nonionized, electrically nonconductive state to an ionized conductive plasma state through the application of a sufficient electrical potential ("channel potential") between opposed cathode and anode running the length of the channel. Removal of this potential causes the gaseous medium to return to an electrically nonconductive state.
When a sufficient electrical potential is applied between anode and cathode, electrons emitted from the cathode ionize gas atoms which then impinge back on the cathode and create more electrons in a rapidly avalanching process that renders the gas conductive, and a plasma quickly develops along the entire channel. This occurs within a microsecond or so after potential is applied. Both cathode and anode are then switched to ground potential and then as the plasma decays in the next several microseconds, the plasma decay processes return all of the surfaces of the channel including the second surface of the barrier to ground potential regardless of the potential they may have had before or during plasma operation.
Therefore, in this state only the pixel's liquid crystal material and thin dielectric barrier separate the conductive surface from ground. Consequently, the potential drop of a pixel is set to a proportion of the difference in electrical potential between the conductive surface and ground. This freezes the pixel gradient in place without regard to the subsequent electrical potential changes of the conductive surface.
Viewed on a larger scale, a PALC display includes a set of channels formed in or on an insulating plate and containing an ionizable gas under a top plate that contacts the tops of the ribs forming the channels and is sealingly connected around the periphery to the insulating plate. Plasma channels are horizontal in these displays, and electrodes are vertical. Each channel defines a row of pixels. Strips of pixel width transparent and electrically conductive material (typically indium tin oxide) extending perpendicularly to the channels face the array of channels across the liquid crystal material. Each strip defines a column of pixels. During operation the channels are sequentially rendered conductive, one at a time. Each time a channel is so rendered, the desired pixel gradients for the pixels of the corresponding row are set by the introduction of an appropriate electrical potential onto each of the conductive strips. This operation occurs many times per second for each channel while the display is in operation.
Parallel electrodes extend along the length of each channel at opposed sides. To avoid differences in electrical potential along the length of the channel cathode during the ionization of the gaseous medium, it is desirable that the resistance per unit length of the electrodes be no more than 2 ohms per centimeter (5 ohms per inch). To achieve this small value of resistance per unit length despite the tiny cross sectional area that is available for the electrodes, highly electrically conductive metals such as gold, silver, copper, or aluminum are used.
Because they are costly, gold and silver are undesirable despite their minimal oxidation during the one hour bake in standard atmosphere that is part of the PALC display fabrication process. Copper oxidizes considerably in this bake and loses conductivity. Aluminum, unfortunately, is less electrically conductive than would be ideal. Copper that is coated with a lower conductivity but oxidation resistant metal, such as chromium or palladium or nickel, provides an electrode of uniformly low resistance per unit length that is sufficiently resistant to oxidation.
In an alternative prior art structure a flat substrate is provided. An odd number of strips of nickel and glass frit particles are deposited on this substrate by way of a screen printing process. First portions of a fine mesh screen are rendered nonporous. The screen is placed over the substrate and a paste of nickel and glass particles suspended in a binder are pressed onto the screen, passing through the porous regions but being blocked by the nonporous regions. The product of this process is baked in air, driving off the organic binder and leaving a deposit of nickel and glass particles that is about 25 microns (1 mil) thick.
In this instance a series of strips of glass and nickel material are formed. To achieve the desired resistance per unit length of electrode, this process may be repeated one or more times to make a thicker and more conductive electrode.
Subsequently, by way of the same process, about 6 layers of insulative material each about 15 microns thick are deposited on every odd strip, counting from either end. This structure defines a set of channels each having a glass and nickel strip extending along the channel center.
This center strip is controlled to be the cathode, whereas the channel defining nickel and glass strips on either side of each channel are controlled to be the anodes. Although the nickel-glass mixture is less conductive than copper or aluminum, the strips have a great enough cross sectional area to be adequately conductive so that they will bear a substantially uniform electrical potential relative to ground over their entire lengths.
Chromium, nickel, or other metals in general, however, if used as the exposed surface for a PALC cathode, are highly susceptible to sputter damage. Sputter damage happens when a positive ion, accelerated by the electric field established between the cathode and anode, collides with the cathode surface and causes an atom in the surface to be dislodged. This process is harmful to the display device, causing the surfaces of the plasma channels to be coated with material which has been sputtered off the cathode. This eventually reduces the panel transparency and eventually increases the conductivity of the second surface of the barrier thus degrading the operating efficiency of the channels, thereby ruining the display.
To avoid sputter damage, the exterior layer of a PALC cathode should be a good emitter of "secondary electrons," and have a high heat of sublimation. "Secondary electrons" are electrons which may be emitted from the cathode in response to the discharge of energy from a nearby (on an atomic scale) "metastable" atom caused by the collapse of a "primary electron" from a high-energy outer valence level to a lower energy valence level. (The presence of an electron that has been elevated to a high-energy outer valence level and is quantum mechanically forbidden to return to a lower energy state is the defining characteristic of a metastable atom.) A similar process with ionized atoms also can cause secondary electrons to be emitted from the cathode and at the same time neutralize the ion.
The probability of a secondary electron being emitted by some cathode surfaces in response to an ion or metastable atom collision is higher than with others. The higher the emission probability is for a given surface, the fewer the number of ions or metastable atoms that are needed to produce sufficient secondary electrons to sustain the plasma. Thus a lower anode to cathode potential is needed to cause the gaseous medium to transition the plasma. This lowered channel potential reduces the kinetic energy imparted to gas ions, which therefore collide with the cathode at a lower speed. Hence, there is a lessened probability of sputter damage at each collision.
"Heat of sublimation" is the amount of energy required to separate an atom from a solid substance (in this case the exterior layer of the cathode). Therefore, a high heat of sublimation equates to a lower probability of sputter damage at each collision.
The exterior coating of the cathode must also not be susceptible to oxidation during the one hour air bake that is an integral part of the PALC display production process.
In addition, any arrangement of materials used to form a cathode sufficient to solve the problems described above would be impracticable unless an economical process is available for realizing the arrangement.
Parent application Ser. No. 08/520,996, filed Aug. 20, 1995, now abandoned which is assigned to the same assignee as the present application, teaches the formation of PALC display cathodes by the electrophoretic deposition of rare earth hexaboride particles over a previously applied conductive and oxidation resistant lead running the length of the channel. This represents a major technological advance, producing a cathode that is sputter resistant and is a good emitter of secondary electrons in comparison with the prior art.
An even greater advantage could theoretically be gained, however, if the coating were electrically nonconductive. To understand why this is so, it is helpful to understand the plasma formation process on an atomic level. When the cathode is energized, it begins to emit electrons in response to ion or metastable atom collisions. Each emitted electron may then cause the formation of a positive ion by colliding with a gas atom and dislodging an electron. Alternatively, an electron and atom collision may form a metastable atom by exciting an electron into an excited energy state from which it is quantum mechanically forbidden to transition to a lower energy state without external perturbations.
The positive ions will be drawn toward the cathode by the electromagnetic field. The metastable atoms will move randomly and may approach the cathode by chance. In either case, when the metastable atom or ion approaches near to the cathode (on an atomic scale), a process known as "Auger excitation," will de-excite the ion or metastable atom and simultaneously excite an electron in the cathode to a higher energy state. If the potential energy released by the ion or metastable is sufficiently large, an electron in the cathode will be excited to a state above "vacuum level."
If the cathode surface is a conductive material, the electric field between cathode and anode will not penetrate the cathode surface. As a result, the excited electron will not be attracted to the cathode surface and be emitted. There is some probability that it still might migrate to the surface due its tendency to move at random so that it might by chance reach the surface and thereby be emitted before it decays to a lower energy state.
If the electrically excited electron were located in an electrically nonconductive region, however, the electric field between cathode and anode could penetrate the region and would attract the electron to the cathode surface where it would be emitted into the plasma. An electrically nonconductive region, which permits the electric field to attract electrons to the surface, thereby causes the cathode to emit more electrons and more efficiently convert the inert gas into a plasma. In addition, many nonconductive materials, MgO and Al.sub.2 O.sub.3 in particular, are very sputter resistant.
The problem with an electrically nonconductive cathode region, however, is that electrons cannot flow through such a region to replace the electrons expelled, and thus these regions quickly become charged and repel electrons from their surface. This problem has been addressed by U.S. Pat. No. 4,663,559 ("Christensen") granted to Christensen which teaches that electrically nonconductive particles having a diameter on the order of 3.5 nm be interspersed with conductive particles on the surface of an electron emitter in a field emission device. The field of application of Christensen is very different from the field of application of the present invention. The cathode of a field emission device is typically contained in a vacuum. In addition, the cathode of a field emission device is typically cone shaped. The tip of the cone, because of its small, compact volume may be subjected to an electric field that is so intense that it bends the energy bands of the material used to a lower energy level. When a nonconductive material is used, as taught in Christensen, the valence bands of the lowest energy electrons can be bent so that at the surface of the tip of the cone, they are below the vacuum level. These electrons are instantaneously emitted. If the nonconductive material is less than about 3.5 nm away from a conductive material, they may be replaced by tunneling and emitted again.
The cathode of a PALC display, however, is contained in a channel filled with a gaseous medium, increasing the potential severity of sputter damage. Because of this the high voltages used for field emission devices may not be used for a PALC display, removing the possibility of bending any of the valence bands of the cathode material electrons below the vacuum level. Also the problems associated with depositing material onto a PALC display cathode are different from those of depositing material onto the small and compact tip of a field emission device. In addition, Christensen fails to completely disclose a method of manufacturing his device.
The use of electrically nonconductive particles that are the order of 3.5 nm in diameter permits replenishment by means of quantum mechanical "tunneling" of the electrons from the conductive particles of the cathode. "Tunneling" is a phenomenon by which an electron in one low energy region that is separated from another low energy region by an intervening thin potential barrier can instantaneously appear in the other low energy region. The probability of an electron tunneling declines exponentially as a function of the width of the intervening potential barrier. Therefore, the smaller the electrically nonconductive particle, the greater the probability that the positively charged region that is created by an electron emission will be neutralized by an electron that tunnels from an adjacent conductive cathode particle after emitting an electron into the gas.